The GOF's Top Tips on How to Have the Best Time!

Introduction

NOTE:
The time stamps present in RXTE FITS tables as generated by XFF are in
TT.

As part of the HEAsoft5.04 patch (February 2001), the GOF released an
improved tool for applying barycentric corrections: FAXBARY. The
previous standard tool for applying these corrections, FXBARY, was
based on inherited routines and on an ephemeris (de200_new.fits) that
does not extend beyond December 31, 2000.

FAXBARY uses the more accurate jplephe.200 ephemeris, extending to
2050. It also incorporates several fine clock corrections that FXBARY
lacked, which improves the absolute time calibration from ~60 microsec
to ~5-8 microsec.

Why Switch from FXBARY to FAXBARY? The
short answer is that FAXBARY is much more accurate than its
predecessor. Here's why:

FAXBARY applies the 16 microsecond delay intrinsic to the PCA.
This was not included in FXBARY.

FAXBARY applies fine clock corrections found in the tdc.dat file.
FXBARY did not apply such corrections. What this means is that the
barycenter corrected times from FXBARY could be up to 60 microseconds
off, depending on the value of the clock corrections it ignored. In
addition, the value of the error varies with time. The plot below
shows the clock correction as a function of mission day.

With
the inclusion of the tdc.dat corrections, absolute timing accuracy of
between 5 and 8 microseconds can be achieved.

The ephemeris that FAXBARY uses includes much more accurate
information
to determine the solar system barycenter. In addition, it is valid
through the year 2050.

(Note that FASEBIN has always included the fine clock corrections from
the tdc.dat file.)

DO's and DON'Ts and other helpful hints:

An input list of orbit files is handled slightly differently
by FAXBARY than by FXBARY. To use more than one orbit file,
either enter them directly at the orbitfiles parameter prompt (as
a whitespace-separated list) or use the typical ftools @filelist syntax,
where "filelist" lists the orbit files one per line. Like FXBARY,
FAXBARY will first examine all of the orbit files and reject any
which do not overlap with the times in the datafile to be corrected.
Note, however, that any list of orbit files MUST be continuous and
not contain any gaps! If this criterion is not satisfied, the FAXBARY
script will print an explanatory error message and exit.

As with fxbary, the value of the TIMEZERO keyword applies
to the BARYTIME column in the output file, thus the barycentric
correction can be derived by simply taking the difference between the
TIME and BARYTIME columns.

If you are comparing results across the 2000/2001 boundary, keep
in mind that the new program is much more accurate than results
from the old program. You may want to reanalyze everything with
FAXBARY if you require consistent accuracy greater than 60 µs.

The RXTE GOF pages provide guest observers with several helpful
explanations of time systems, time stamps, and coversions. These
include: